The introduction of high activity Ziegler-Natta catalyst systems has led to the development of new polymerization processes based on gas phase reactors as disclosed in U.S. Pat. No. 4,482,687, issued Nov. 13, 1984. These processes offer many advantages over bulk monomer slurry processes or solvent processes. They are more economical and inherently safer in that they eliminate the need to handle and recover large quantities of solvent while advantageously providing low pressure process operation.
The versatility of the gas phase fluid bed reactor has contributed to its rapid acceptance. Alpha-olefin polymers produced in this type of reactor cover a wide range of density, molecular weight distribution and melt indexes. In fact new and better products have been synthesized using single- and multiple-, or staged-, gas phase reactor systems because of the flexibility and adaptability of the gas phase reactor to a large spectrum of operating conditions.
Conventional gas phase fluidized bed reactors used in polymerizing alpha-olefins have a cylindrical shaped fluidized bed portion and an enlarged, tapered-conical entrainment disengaging section, sometimes referred to as the expanded section or transition section. The enlarged entrainment disengaging section is employed to minimize the quantity of fine powder, or fines, carried out of the reactor. Fines can adversely affect properties of the polymer product. Also, fines can be transported from the reactor into the recycle system by the fluidizing gas. Additionally, during polymerization a phenomenon known as sheeting can occur. Sheeting is the adherence of fused catalyst and resin particles to the walls of a reactor, particularly in the expanded section of the reactor. When the sheets become heavy, they can fall off the walls and plug the product discharge system or clog the distributor plate. These sheets can also contribute to product quality problems by increasing the gel level in end-use products such as plastic containers and films. Sheeting and fines accumulations are collectively referred to as solid particle build-up.
Conventionally, to prevent sheeting from affecting these and other parts of the reactor system, as well as the final polymer product, the reactors are shutdown periodically and the walls are cleaned. When a reactor is down for cleaning, it is typically hydro-blasted using water under high pressure to remove sheets and fines build-up. Since water is a poison, as well as air, the reactor must be purged to remove these poisons and the reactor must be dried. This process is both time consuming and costly. As a result, significant savings can be obtained with the prevention of a single shutdown.
It is also conventional practice to maintain the level of the fluidized bed a few feet below the neck of the expanded section to avoid the accumulation of fines in the expanded section of the reactor. Thus, the volume of the fluidized bed, and therefore the amount of polymer in the reactor is fixed. If one were able to lower the fluidized bed level and maintain a high production rate, the residence time of the polymer would be greatly reduced and the flexibility of the reaction system would be enhanced.
In addition, during the operation of the gas phase fluidized bed polymerization reactor system, there are times when it would be desirable to adjust the powder inventory and/or solids residence time. Catalyst productivity and polymerization rate are affected by the residence time of the solids such as resin and catalyst in the reactor. Control of catalyst productivity and polymerization rate by adjusting residence time would be a desirable method for controlling reactors that are operated in sequence (i.e., staged reactors) to produce products such as bimodal polymers or copolymers. In these types of polymerization processes, control of the proportion of polymer made in each reactor plays a role in determining the properties of the final product and its property consistency.
For product grade transitions, it typically requires about one to three bed turnovers. By altering the fluidized bed volume and therefore the resin particle residence time, the time to achieve the number of turnovers could be lessened. Accordingly, the amount of off-grade polymer product generated during reactor start-up and during grade changes could be reduced.
Accordingly there is a need to improve reactor operation and to improve product quality by reducing sheeting and the accumulation of fines in the expanded section of a reactor as well as in other areas of the reactor system.